The present invention relates to monolithic analog signal switches, and more particularly to high voltage analog switches and control circuitry constructed on a dielectrically isolated substrate. A typical application of such a device is in an instrument used for measuring small currents accurately over a wide dynamic range of voltages. Such a device might be used in a bridge measurement circuit and more specifically in a wiring analyzer system where several thousand of the switching devices are connected in parallel to form a switching matrix. An automatic test equipment (A.T.E.) application may require floating, high speed, high voltage switches.
Devices constructed on a dielectrically isolated substrate, known as a d.i. substrate, have specific advantages over devices constructed using conventional junction isolation technology in certain classes of monolithic semiconductor devices. Whereas junction isolated devices on a substrate always combine to form undesired parasitic devices between active devices and the substrate (such as bipolar transistors, MOSFETs and SCRs), a d.i.-based device does not. Components constructed on a d.i. substrate are isolated in individual wells of monocrystalline silicon surrounded on all sides by a shell deposit of silicon dioxide and supported by a polysilicon backing. For high voltage applications, therefore, a device employing d.i.-based components can be more tightly packed than a comparable junction isolated device.
D.i. substrates are expensive because of the extensive processing required to prepare the stock substrate. It is therefore desirable to make more efficient use within an isolation well of a d.i. substrate.
It is advantageous in an analog switching device that the control components be closely associated with the switching components for performance, reliability and cost reasons. In the past, control components have been constructed in monolithic form, such as a gate array, on dedicated die, and then are combined in hybrid packaging with switching components, which were either as a monolithic or hybrid implementation. Efforts have also been made to construct switching devices combining control and switching components on a monolithic substrate. For example, junction isolation devices are built by Mitel, Inc. of Boca Raton, FL for use in low voltage and/or low current applications such as cross-point switching in a telephone network.
It has been found that monolithic junction isolated switching devices are unsatisfactory in applications where high accuracy and/or high voltage are critical parameters. It is critical for example that there be no current leakage between active components or through the substrate which could effect measurement accuracy. What is needed is a reliable, accurate and relatively inexpensive monolithic switching device capable of switching operation at voltages and currents greater than is used for conventional monolithic-based control applications, and more particularly in voltage ranges above about 20 volts to about 300 V and currents above about 10 mA to about 1 Ampere.
Attempts have been made to build and sell d.i. based analog switching arrays since about 1985. For example, Telmos, Inc., of Sunnyvale, CA, now defunct, manufactured a monolithic analog switch and control array on a d.i. substrate for Electronic Systems and Programming, Inc. of Camarillo, CA, now Test Systems/ESP, a subsidiary of Xebec, Inc., the parent company of the assignee of the present invention. The device was described in EDN Magazine. The device was necessarily relatively large and consequently not cost effective. As a result, the product was a commercial failure and resulted in the failure of Telmos, Inc.
AT&T Technologies of Reading, PA recently introduced a high voltage gate array on a d.i. substrate. Currently, however, AT&T Technologies is providing custom design services for d.i.-based switching devices.